The per capita demand for cellulosic fibers in textiles is expected to increase from 3.7 to 5.4 kg in the next 15 years. With the Earth's population estimated to grow from 6.9 to 8.3 billion in the same time period, it is expected that cotton, a major source of cellulosic fibers, will not meet market demand due to the estimated production of only 3.1 kg per capita in 2030, based on anticipated shrinkage of cotton growing area (Hauru et al. 2013). Therefore, producing man-made cellulosic-based fibers, such as viscose, cellulose acetate, etc., from dissolving pulp will be a necessary alternative to make up this large market shortage of at least 1.7 kg per capita for native cellulosic fibers. Dissolving pulp fibers are commercially produced using either sulfite pulping or hot-water pre-hydrolysis coupled with kraft pulping, both developed in the 1950's. The main problems with sulfite pulping are chemical recovery and environmental concerns due to SO2 air emissions. The metal base in sulfite pulping, excepting magnesium, cannot be recovered. Pre-hydrolysis with kraft pulping is very expensive in terms of energy. Chemical recovery in kraft pulping is commercially practiced using the Tomlinson recovery boiler, but is capital intensive. The hemicellulosic sugars from hot-water pre-hydrolysis are often discarded to save energy, which can create substantial biochemical oxygen demand (BOD) problems.
Cellulose nanomaterials have attracted great attention recently for their unique optical and mechanical properties (Moon et al. 2011; Zhu et al. 2016). Most of the published research, however, has been focused on cellulose nanomaterials produced using bleached fibers that do not contain lignin. Lignin is relatively hydrophobic, which can be beneficial for certain applications. Unfortunately, only a few studies have reported the production of lignin containing cellulose nanomaterials from commercial unbleached chemical pulps using direct mechanical fibrillation, which is energy intensive and does not produce surface functional groups which aid dispersion (Rojo et al. 2015; Spence et al. 2010). A study on the production of lignocellulose nanomaterials directly from wood is reported as a patented process by American Process, Inc. at high temperatures using an organic solvent solution of concentrated ethanol and sulfur dioxide (Nelson et al. 2015).
Nano sized particles have attracted great interest due to their large specific surface areas and shape dependent properties for a variety of potential applications (Xia et al. 2009). Organic nanoparticles (Kamaly et al. 2016; Mavila et al. 2016; Reisch and Klymchenko 2016), especially those derived from biodegradable and benign natural biopolymers, such as cellulose, chitin and DNA, are more attractive from a sustainability point of view. Lignin, the second most abundant natural polymer from a plant biomass cell wall, has so far found limited economical utilization other than as a boiler fuel through combustion in pulp mills (Duval and Lawoko 2014; Upton and Kasko 2016). With rapid advances in nanotechnology, lignin, as a renewable and abundant biopolymer, has gained growing interests in the nanotechnology field (Frangville et al. 2012; Nair et al. 2014). Lignin nanoparticles (LNPs) have potential applications in developing novel and biodegradable materials and advancing biotechnologies (Jiang et al. 2013; Qian et al. 2014; Richter et al. 2015; Ten et al. 2014).
The commercial applications of LNPs through industrial processing, however, are impeded by the difficulties in economical production from the plant cell wall. Almost all of the existing methods for the production of LNPs use commercial technical lignin which requires dissolution in solvents, such as ethylene glycol, acetone, tetrahydrofuran (THF), or N,N-dimethylformamide (DMF), followed by either acidic precipitation (Frangville et al. 2012; Richter et al. 2016), hexane precipitation (Qian et al. 2014), dialysis (Lievonen et al. 2016), or atomization and drying (Ago et al. 2016). The use of organic solvents such as ethylene glycol and THF is an environmental concern and increases LNP cost for solvent recovery. Also, the LNP properties are affected by the original feed lignin sources generated from various pulping processes.
Hydrotropic chemistry using concentrated aromatic salts as solvents for solubilizing a range of hydrophobic compounds was discovered in 1916 by Neuberg. Its application for fractionation of lignocellulosic biomass was first practiced by McKee (McKee 1943). For pulping poplar using 30-40 weight percent (wt %) aqueous sodium xylenesulfonate liquor, a reaction of temperature of 150° C. for 11-12 hr was needed to obtain a cellulosic solids yield of 52% (McKee 1946). There are many hydrotropic agents that can be used to dissolve lignin (Procter 1971). The most used salts were sodium salicylate and xylenesulfonate, cumenesulfonate. Sodium xylenesulfonate was found to have very strong hydrotropic activity at 30 wt % and only required a 3-time dilution to lose its hydrotropic properties and, thus, precipitate lignin (Robert 1955). There have been numerous studies on hydrotropic pulping since its invention (Gromov and Odincov 1959; McKee 1954; Procter 1971) including using additives (Kalninsh et al. 1967; Nelson 1978). However, the processes were never commercialized due to low pulp yields, poor pulp mechanical properties, and very long cooking times. Moreover, the processes were not suitable for pulping softwoods, due to insufficient delignification (Procter 1971). Recently, hydrotropic pulp was found to be enzymatically digestible for sugar production (Korpinen and Fardim 2009). To reduce reaction time, additives such as formic acid and hydrogen peroxide were used (Gabov et al. 2013). A recent study included the characterization of lignin from modified hydrotropic processes used for subsequent sugar production (Gabov et al. 2014). The utilization of hydrotropic lignin, however, has remained limited (Kalninsh et al. 1962; Procter 1971).